1. Field of the Invention
This invention relates to methods for mixing liquid waste in lifting stations. More specifically, this invention relates to methods for breaking up grease and other solid and semi-solid accumulations in liquid waste lifting stations.
2. Description of the Related Art
Wastewater systems typically use gravity to transport liquid waste from homes and businesses to provide wastewater treatment at a central facility. For cities that have many changes in elevation, lift stations are used to pump liquid waste, such as storm water and sewage, from low-lying areas to areas of higher elevation for further transport and treatment.
A wastewater lift station typically comprises a concrete well that is fitted with several pumps, often of submersible design. Lift station design further includes level-sensing probes, valves and pressure sensors, and may also include a stand-by generator. In operation, the lift station serves as a temporary holding tank for liquid waste, the waste being periodically pumped from the lift station to higher elevations. Liquid waste is transported by the upstream wastewater system to flow into the lift station well. When the amount of liquid waste in the well reaches a pre-determined high level, the lift station's pumps are engaged to pump liquid waste from the lift station to the downstream wastewater system. The pumping progresses and liquid waste is emptied from the lift station until the amount of liquid waste in the well falls to a pre-determined low level, at which point the lift station pumps are disengaged.
Semi-solid, water immiscible materials, such as cooking oil, lubricants and grease, as well as light plastics, are common components of the liquid waste slurry entering wastewater lift stations. As wastewater is held in a lift station, these materials, along with scum from calcium and magnesium soaps tend to coagulate and adhere to lift station walls and equipment. Because many of these materials are less dense than water, they tend to float on the surface of the waste liquid, accumulating particularly with pronounced thickness on walls and equipment near the high liquid level in the lift station. As is known by those in the art, the level-sensing probes used in most lift stations are simply floats, detecting liquid level by floating on the surface of the wastewater. Accumulations of insoluble material within the lift station can restrain the floats below the surface of the waste water, leading to improper operation of the lift station and, in extreme cases, lift station overflow when the float is unable to detect that the liquid waste in the well has reached and exceeded its pre-determined operational high level. Furthermore, over time, the amount of such accumulated insoluble material can actually reduce the holding capacity of the lift station and, because large pieces of such material can break off and become lodged in lift station equipment such as valves and pumps, the presence of such large accumulations may adversely affect lift station operation and damage lift station equipment.
A number of approaches have heretofore been used to break up and reduce or eliminate the accumulated deposits of immiscible material in lift stations. One approach for dealing with such accumulated deposits is by direct mechanical removal, emptying the tank of waste liquid followed by manual cleaning of accumulations by appropriately garbed personnel. Drawbacks in such approaches include the fact that the station must remain inoperative during the time it is emptied and cleaned. Other drawbacks simply follow from aesthetic and hygienic challenges of placing personnel inside wells that have held raw sewage.
Another approach to remediate semi-solid accumulations in lift stations has been to use pumps of various designs to generate strong currents in the liquid waste within the lift station, in order to effect the mechanical break-up of accumulated deposits. This approach has the drawback that pumps providing sufficiently strong currents for such purposes consume considerable energy. Furthermore, the pumps, being electromechanical devices, are inevitably subject to significant maintenance and repair costs.
Another approach gaining wide acceptance for removing or reducing accumulations of immiscible material in lift stations is biological augmentation, whereby specialized aerobic, grease-metabolizing bacteria are introduced into the tank wastewater. When properly applied, these organisms can break down large lift station grease deposits into smaller pieces which can then be pumped in a slurry with other liquid waste into the downstream wastewater system by lift station equipment. Because the microorganisms used for biological augmentation are aerobic, the availability of oxygen in the wastewater can be a limiting factor on the effectiveness of the process. For this reason, effective biological remediation often requires the addition of aerating diffusers to the lift station tank along with necessary supply lines and source of pressurized gas to supply needed oxygen.
Biological remediation is subject to several additional limitations. Use of too high a concentration of bacteria in the lift station wastewater, while leading to fairly rapid grease deposit break-up, can result in large quantities of small pieces of undigested grease transferred to the downstream wastewater system, leading to downstream clogging and excessive foaming at the wastewater treatment plant. While use of lower concentrations of bacteria may actually result in more complete digestion of accumulated grease with fewer downstream problems, effective remediation by lower bacterial concentrations takes much longer, potentially leaving the lift station partially or fully inoperative for a longer period of time. Further, even when such biological augmentation is optimized, light plastic immiscibles deposited in the lift station, such as condoms, plastic wrap and the like remain unaffected.